Exhaust gas recirculation (EGR) methods and devices for use with internal combustion engines are known. Most EGR systems include at least one EGR valve and optionally at least one EGR cooler connected in series between an exhaust system and an intake system of an engine. A typical EGR system is capable of mixing a portion of exhaust gas generated by the engine with fresh air entering the engine. Introduction of exhaust gas into an intake air stream of the engine displaces oxygen in the intake stream to yield a lower flame temperature of combustion, and thus, lower nitrous oxide (NOx) emissions.
Some engines, especially compression ignition or diesel engines, use coolers that cool the portion of exhaust gas being recirculated. The cooled exhaust gas has a lower latent heat content and can aid in lowering combustion temperatures even further. In general, engines using EGR to lower their NOx emissions can attain lower emissions by cooling the recirculated exhaust gas as much as possible.
Exhaust gas constituents in the exhaust gas being recirculated on an engine with EGR often present problems when the exhaust gas is cooled below a condensation temperature of those constituents. Various hydrocarbons will typically condense onto engine components and present issues such as sluggish performance or even sticking of moving parts. These issues are especially evident when an engine starts under cold ambient conditions, when most engine components are cold and exhaust gas constituents condensate more readily onto the engine components.
Most engines in the past have attempted to cope with the problem of condensation of exhaust gas constituents by delaying initiation of EGR under cold start conditions, or limiting the amount of exhaust gas being recirculated, or limiting the amount of cooling applied to the recirculated exhaust gas in an effort to minimize the degree and amount of condensates. Such measures, although effective in increasing the service life of engine components and decreasing the likelihood of failures, are insufficient in addressing the impact they have on the emissions generated by the engine. The more delayed the initiation of EGR becomes, or, the limited amount of cooling of the exhaust gas, directionally qualitatively increase the emissions generated by the engine.
Some engine designs cope with the issue of condensation by placing the EGR valve upstream, or on the “hot side”, of the EGR cooler. This placement of the EGR valve ensures that the valve will not be exposed to cooled exhaust gas, and thus be immune to the condensation effects that result from the cooling, but these configurations have disadvantages. One disadvantage is the increased flow orifice size required for the EGR valve because the gas passing therethrough is at a high temperature and low density. Increased mass flow rates of exhaust gas through the EGR valve in these systems inevitably leads to large EGR valves. Also, placement of the EGR valve on the hot side of the EGR cooler exposes the EGR valve to high temperatures. With most EGR valves having electronic components and precise mechanical components associated therewith, therefore, the increased service temperature of valves requires use of active cooling systems for them, and also use of exotic materials for the mechanical parts, both of which increase the cost and complexity of these valves and of the engines that use them.